Many wireless devices currently operate on multiple frequency bands. These bands may be widely spaced in the frequency spectrum. For example, existing CDMA (Code Division Multiple Access) cell phones can operate in the 800 MHz and 1900 MHz bands. Operation on other bands is also foreseeable as mobile networks adopt new wireless technologies, such as WiFi and WiMax technologies for data transmission, which communicate at other frequencies, such as 2.4 GHz, 2.5 GHz, 3.5 GHz, or 5.8 GHz.
In most cases, it is desirable for antennas to have a high ratio of radiated power to incident power at all frequencies of operation, thus reducing wasted energy during both transmit and receive operations and minimizing potentially damaging power reflected back through the feeding terminal of the antenna. This ratio consists of two components: the antenna efficiency, eradiation, and a factor, X, relating power entering the antenna, Pentering, and power incident on the antenna, Pincident:
                    P        radiated                    P        incident              =                  e        radiation            ·      X        where            e      radiation        =                                        P            radiated                                P            entering                          ⁢                                  ⁢        and        ⁢                                  ⁢        X            =                                    P            entering                                P            incident                          =                              [                          1              -                                                                                      Γ                    terminal                                                                    2                                      ]                    .                    
Γterminal is the reflection coefficient between the feeding terminal and the antenna. A smaller value of Γterminal represents less power reflected and more power that has entered the antenna. Since the radiation efficiency depends on the antenna layout and materials, it is generally fixed for a given design. Therefore, to increase the ratio of radiated power to incident power, X may be reduced by minimizing the reflection coefficient by matching the circuit.
The system is perfectly matched when Zin (the impedance of the antenna) is equal to Z0 (the impedance of the feed network), thus making the reflection coefficient zero as shown by the following relation:
      Γ    terminal    =                              Z          in                -                  Z          0                                      Z          in                +                  Z          0                      .  
When Γterminal=0, all power incident on the antenna from the feed terminal is accepted by the antenna. Since standard feed networks typically present only real impedance, the antenna should ideally present a zero reactance at the frequency of operation and a resistance equal to that of the feed network to be perfectly matched. To meet this goal, therefore, antennas are generally sized to resonate (i.e., present zero reactance) near or at the frequency band or frequency bands of operation.
A straightforward way of operating in multiple bands is to use more than one antenna, each resonant at a different frequency. For example, U.S. Pat. No. 7,019,696 to Jatupum Jenwatanavet, “Tri-band Antenna,” which is hereby incorporated by reference in its entirety, describes a system in which three antennas are combined to operate at three different frequency bands, and each antenna is sized for one particular frequency.
The ongoing miniaturization of wireless devices indicates that the antennas used in portable cell phones, PDAs, network cards, laptops and the like, will have to be of a relatively small size to be capable of being integrated into the devices.
The multi-antenna arrangement described in U.S. Pat. No. 7,019,696 and others like it that use multiple antennas to achieve multi-band operation replicate elements common to each antenna, and these replicated elements take up space, which may be unacceptable in some applications.
Several attempts have been made to design antenna structures that resonate at multiple frequencies. For example, U.S. Pat. No. 6,611,691, to Guangping Zhou, Michael J. Kuksuk, Robert Kenoun, and Zafarul Azam, “Antenna adapted to operate in a plurality of frequency bands,” which is hereby incorporated by reference in its entirety, discloses a whip antenna that can be extended to two lengths to achieve two different resonant frequencies. However, such an antenna is too large to be effectively integrated inside many wireless devices, and it would also be cumbersome to operate, since some mechanical system would be needed to extend the antenna to change it from one mode of operation to the other. The antenna would typically extend outwardly from the device and could easily break off.
To meet the sizing requirements for portable wireless devices, many varieties of conventional compact antennas have been developed, including bent antennas. Bent antennas include bends along the length of the antenna, thereby increasing the electrical length of the antenna within a given area.
A popular design for compact antennas is the Inverted-F Antenna (IFA), which is described in H. Y. David Yang, “Printed Straight F Antennas for WLAN and Bluetooth”; IEEE Antennas and Propagation Society International Symposium, 2003, 22-27 Jun. 2003, Volume: 2, page(s): 918-921, which is hereby incorporated by reference in its entirety.
An example of a conventional IFA 20 is shown in FIG. 1. This antenna 20 is essentially a bent monopole, except that the grounding point 28 and feed point 22 are separated. A signal is fed into the feed point 22 of the antenna 20 through a connector (not shown). The antenna 20 has a first line length 25 that splits into a first branch 24 and a second branch 26. The end of the second branch 26 is grounded and acts as the grounding point 28 of the antenna 20. The second branch 26 includes a bend that allows the electrical length of the second branch to be increased without significantly increasing the area occupied by the antenna 20.
The impedance presented by a monopole antenna at its feed point 22 depends on the location of the feed point 22 relative to the ground point 28, as illustrated in FIG. 2. If the feed point 22 is moved closer to the ground point 28, effectively shortening the second branch 26 and increasing the length of the first branch 24, the impedance of the antenna 20 to a signal source 30 at the feed point 22 decreases. Alternatively, if the feed point 22 is moved further from the ground point 28, effectively increasing the length of the second branch 26 and shortening the first branch 24, the impedance of the antenna 20 to the signal source 30 at the feed point 22 increases. Thus, by appropriately positioning the feed point 22 relative to the ground point 28, a higher or lower impedance will be seen by the signal source 30 and proper matching can be achieved to increase the ratio of radiated power to incident power. Note that this property also applies in general to other types of antennas.
Although most IFAs are used at a single frequency band, some multi-band designs have been disclosed. U.S. Pat. No. 6,819,287 to Jonathan Lee Sullivan and Douglas Kenneth Rosener, “Planar inverted-F antenna including a matching network having transmission line stubs and capacitor/inductor tank circuits,” which is hereby incorporated by reference in its entirety, describes an IFA capable of selective dual-band operation; these two bands, however, are two natural resonances of the full antenna length, and thus this arrangement cannot be applied to systems where the desired frequency bands are not as such.
Discontinuities in an antenna, including changes in impedance, materials, and geometry, also create additional resonances. Beyond simply reducing the antenna footprint, bends are particularly useful as discontinuities because energy is reflected at each discontinuity in the line caused by each bend, creating a null in the standing wave pattern and creating an additional resonance. The antenna would resonate at the total electrical length of the antenna and also at the electrical lengths measured from the source to each discontinuity. Therefore, multiple resonances at frequencies that are not related to the natural resonance of the total length of the antenna can be obtained using multiple bends.
U.S. Pat. No. 6,903,686 (hereinafter referred to as the '686 patent), to Scott LaDell Vance, Gerard Hayes, Huan-Sheng Hwang, and Robert A. Sadler, “Multi-branch planar antennas having multiple resonant frequency bands and wireless terminals incorporating the same,” which is hereby incorporated by reference in its entirety, uses the multi-resonant property of an Inverted-F Antenna; the design is shown in FIG. 3.
In the design shown in FIG. 3, the antenna 120A includes a straight portion 121c1 and a curved portion 121c2. The antenna 120A is driven at a feed point 161s by a source and is grounded at a ground point 161g. 
The '686 patent mentions that the antenna 120A can resonate at multiple frequencies (800 MHz, 900 MHz, 1800 MHz and/or 1900 MHz). Use of the antenna's natural multiple resonances will result in zero input reactance at these frequencies. However, the resistance, as specified by the location of the feed point 161s relative to the ground 161g, will be optimized for the primary design frequency only, and thus, the efficiency is likely to be lower at other frequencies of operation. For instance, FIG. 9 of the '686 patent shows the high-band VSWR result to be approximately 1.2. From the relation
  VSWR  =            1      +                      Γ                            1      -                      Γ                    The reflection coefficient magnitude is found as|Γ|=0.09The antenna impedance can then be found, assuming a 50Ω feed network impedance:
                  Γ              =                            Z          L                -                  Z          0                                      Z          L                +                  Z          0                          0.09    =                            Z          L                -        50                              Z          L                +        50                        Z      L        ≅          59      ⁢      Ω      
A similar calculation for the low-band result shows that the impedance of 75Ω seen at this lower frequency is not as well matched to the feed network impedance, thereby decreasing the efficiency. The mismatch at one of the operating frequencies is a drawback of this design.
Multiresonant designs of this type have an additional problem; because the antenna may receive signals at all natural resonances, additional circuitry may be required in the wireless radio receiver to filter undesired signals.